The present invention relates to a magnetic recording disc formed of a ceramic substrate, and a process for producing such magnetic recording disc. More particularly, it is concerned with a magnetic recording disc formed of a ceramic substrate provided thereon with a glass coating layer having an improved surface roughness, substantially no air bubbles, and being free from strain due to machining; and a process for its production.
In general, the magnetic recording disc is required to have the following properties:
(1) that it is excellent in its stable floating of the magnetic head and stable recording characteristics in association with floating height of the magnetic head of as low as 0.3 micron (.mu.m) or below; PA1 (2) that it is free from any defects in the magnetic thin film formed on the surface of the substrate member; PA1 (3) that it has mechanical strength sufficient to withstand machining, polishing or high speed rotation during its use with the least flexure; and PA1 (4) that it has sufficient corrosion-resistance, weather-resistance, and heat-resistance. PA1 (a) a substrate member composed of an alumina-based ceramic material containing therein micropores with a size of 5 .mu.m or below and having a relative theoretical density of 90% or higher; PA1 (b) a glass coating layer of a thickness in a range of from 3 to 200 microns (.mu.m), having a surface roughness (R.sub.z) of 180 angstroms or below, and with the surface thereof being free from micropores and strain; PA1 (c) a magnetic thin coating film formed on the outermost surface of said glass coating layer; and PA1 (d) a protective coating film formed on said magnetic thin film. PA1 (a) forming a glass coating layer to a thickness in a range of from 5 .mu.m to 220 .mu.m on the surface of a substrate of an alumina-based ceramic material containing therein micropores with a size of 5 .mu.m or below and having a relative theoretical density of at least 90%; PA1 (b) then, subjecting said glass coating layer on said substrate to mechanochomical polishing to render it to have a layer thickness of 3 to 200 .mu.m, to be substantially free from micropores and strain, and to have a surface roughness (R.sub.z) of 180 angstroms or below; and PA1 (c) further coating the top surface of said glass coating layer with a magnetic thin coating film and a protective coating film.
Heretofore, aluminum alloys have been used as the substrate members for the magnetic recording disc. However, owing to the crystal anisotropy in the aluminum alloy materials, defects in these materials as well as non-metallic inclusions existing therein, such non-metallic inclusions are apt to remain on the surface of the aluminum alloy substrate in the form of protrusions, or they tend to separate away from the surface thereof to give rise to indents, even after the machining and polishing, with the consequence that the surface roughness (R.sub.z) of about 200 angstroms at the most could only be attained notwithstanding extensive polishing work having been carried out on it. Such surface state of the material having protrusions, indents and undulations makes it difficult to realize the high density recording and the low floating height, hence it does not provide a satisfactory substrate for the high density magnetic recording disc having excellent reliability.
That is to say, the quality in the surface state, the shape and the precision in the machining of the magnetic disc substrate members directly affects the displacement of the magnetic recording disc, the speed acceleration component for the magnetic recording disc, the signal errors in the magnetic recording medium, and so forth.
Since the aluminum alloys are metallic materials, they have a Vickers' hardness on the order of Hv 100 (in the case of ceramics, it is Hv 600 or more) and a bending strength on the order of 1,000 kgf/cm.sup.2 (in the case of ceramics, it is 4,000 kgf/cm.sup.2 or more). For that reason, as the recording density increases, more stringent requirements are imposd upon the shape and the dimensional precision thereof in respect of scratch, flaw, surface flatness and undulation, on account of which more difficulty is accompanied in its machining.
Also, in the case of the aluminum alloy substrate members, there would be a possibility of abrasive particles being packed in the indents at the surface part of the substrate member at the time of its machining by use of abrasive particles, which entails another problem; and, moreover, in order to increase the surface-corrosion-resistance and the weather-resistance as well as to prevent the substrate member from its surface contamination, a great deal of care should be taken to secure cleanliness and rust-prevention as well as to avoid contamination, etc. in the production steps of the substrate inclusive of the lathe-turning and the polishing works, as well as in the storage period thereof.
For the purpose of improving the aluminum alloy substrate members, there has so far been proposed a method, in which a film having a high hardness is formed on the surface thereof. As an example, there has been adopted a method, in which an alumite layer is formed on the surface of the aluminum alloy substrate to increase its hardness, thereby improving its abrasive machinability. However, traces of impurities (such as Fe, Mn, Si) contained in the aluminum alloys precipitate as intermetallic compounds during formation of the alumite, which are liable to bring about the surface indents after the alumite treatment. With a view to avoiding such unfavorable phenomenon to take place, there has been practised a method, in which an undercoating layer of Ni, P, etc. is formed on the surface of the substrate member to finish its surface.
It is extremely difficult to attempt further purification of the aluminum alloy matrix from the point of view of its production process. In addition, the aluminum alloys raise a handling problem from the standpoint of their corrosion-resistance and cleanliness.
Furthermore, formation of a thin film magnetic recording medium by plating or sputtering onto the surface of the aluminum alloy poses problems in connection with the occurrence of chemical reactions and diffusion between the aluminum alloy and the magnetic thin film. There is also a problem such that, due to a heat treatment applied to the magnetic film upo its coating, deformation is caused to the aluminum alloy substrate, which would simultaneously bring about increase in the surface vibration and acceleration at the time of rotation of the substrate member (disc).
There has also been proposed a method, in which an oxide such as SiO.sub.2, Al.sub.2 O.sub.3 or the like is formed on the aluminum alloy substrate by sputtering. This method, however, is disadvantageous in that the adhesive force between the aluminum alloy substrate and the sputtered oxide film is weak, and also has a problem in its productivity.
Alumina-based ceramic materials have become widely used in various fields due to their superiority over the aluminum alloy materials in respect of the heat-resistance, wear-resistance, weather-resistance, insulatin, and mechanical strength. And, in order to fulfil the requirements for such magnetic disc substrate member, there is a strong demand on an alumina-based ceramic substrate member to have a surface and coated layer thereof free from any micropores and strain in association wth necessity for forming the thin film magnetic medium on the substrate member surface, and with the thinning and high densification of the recording medium.
In general, as the methods for producing the ceramic substrate member, there have been known the single-crystallization method; method, wherein the substrate is sintered after it has been shaped by metal mold forming, rubber press forming, doctor blade forming, etc.; and further the hot-pressing (HP) process as well as the hot isostatic pressing (HIP) process for obtaining the ceramic substrate having much more increased density. However, the single-crystallization method is not only high in the production cost, but also is difficult to produce a substrate having a large diameter. While, on the other hand, the hot isostatic pressing process and the hot pressing process are capable of producing highly densified substrate, use of such ceramic substrate for the magnetic recording disc raises certain problems in its operational reliability such as occurrence of drop-outs, head crush, and so on due to minute surface defects (these methods are still liable to leave micropores of 5 .mu.m or below in the substrate) of the resulting substrate.
In general, the mechanochemical polishing method, which is applicable to the magnetic disc substrate member, etc. as a surface polishing method, has been known to be capable of precisely finishing the surface of silicon substrates, GGG crystals, ferrite, and so on without deteriorating the surface physical properties thereof. However, when this mechanochemical polishing method is applied to ceramic materials, in which micropores exist, it renders micropores to be exposed to the surface of the ceramic material with the result that such ceramic materials are not eligible for the magnetic disc substrate members, on which a thin film magnetic medium is to be coated. On the other hand, when the mechanochemical polishing process is applied to the alumina-based ceramic substrate, there arise problems such that exposure of micropores to the surface thereof and step-difference between crystal grains occur simultaneously due to difference in the rate of chemical erosion on the surfaces of constituent materials or crystal grains.